The invention relates to a process for working up a raw ester of an esterification reaction catalyzed by a metallic esterification catalyst.
Esters of phthalic acid, adipic acid, sebacic acid or maleic acid find wide use in coating resins, as constituents of paints and especially as plasticizers for polymers.
It is known that carboxylic esters can be prepared by reacting carboxylic acids with alcohols. This reaction can be carried out autocatalytically or catalytically, for example by means of Brønsted or Lewis acids. In many cases, metal compounds are used as catalysts, such as the alkoxides, carboxylates and chelate compounds of titanium, zirconium, tin, zinc and aluminum.
Even though the catalytic properties of these metallic catalysts are satisfactory, the removal of the catalyst residues from the esterification products presents difficulties. For purification, the raw esters are generally first admixed with alkali metal hydroxides to remove unconverted or incompletely converted acid (partial esters), and the free alcohols are removed by steam distillation. After brief vacuum distillation to dry the product, the catalyst residues are then removed by filtration. Since the catalyst residues are generally of slimy, gel-like consistency, filtration is usually possible only with the aid of filtration aids, for example activated carbon, wood flour or kieselguhr. Nevertheless, such a filtration is still associated with serious disadvantages: long filtration times are required and the yield of ester is reduced because large amounts of products are retained in the filtercake.
DE 194 53 59 discloses a process for working up raw plasticizers, which has the following successive steps: (i) the residue acid in the raw plasticizer is neutralized with alkaline substances (e.g. 25% sodium hydroxide solution); (ii) the free alcohols in the raw plasticizer are removed by means of steam distillation; (iii) the product is cooled to temperatures below the boiling point of the water at the particular pressure; (iv) at least 0.5% by weight of water, based on the product to be worked up, is added; (v) the mixture of water and product to be worked up is stirred intensively at temperatures below the boiling point of the water at the particular pressure for at least 15 minutes; (vi) the water added is removed by vacuum distillation; (vii) the plasticizer is filtered.
When the sodium hydroxide solution is added under the conditions specified, a significant portion of the water supplied with the aqueous alkali evaporates immediately, and so solid sodium hydroxide precipitates out. Solid sodium hydroxide reacts significantly more slowly than dissolved NaOH. In addition, the precipitation leads to deposits on pipelines and vessels, which necessitate frequent cleaning.
DE 23 30 435 describes a process for working up raw esters, in which the raw ester at a temperature of 140-250° C. is neutralized under reduced pressure simultaneously with aqueous solutions of alkali metal or alkaline earth metal hydroxide, and subjected to a steam distillation by admixing with water under reduced pressure, then dried and the solid constituents formed are filtered off. The pressure and the rate of water addition should be regulated such that the water added evaporates rapidly.
Under the process conditions under which added water evaporates immediately, solid alkali metal or alkaline earth metal hydroxide can precipitate out, which leads to the above-described disadvantages. Since solid hydroxide reacts significantly more slowly, high base excesses are sometimes required for complete neutralization.
EP 1 300 388 discloses a process for preparing carboxylic esters, wherein the excess alcohol is removed after the esterification reaction, and the raw ester thus obtained is neutralized by addition of base and is then filtered. The alcohol is removed by at least one steam distillation and the base is added during a steam distillation. The alkali is to be sprayed into the reaction mixture at the bottom. As a result of the high temperature, the water evaporates. As a result of low rates of metered addition of the alkali, side reactions, for example the hydrolysis of the esters, are to be minimized. However, this has the disadvantage of long neutralization times and/or low throughputs.
U.S. Pat. No. 5,434,294 describes a process for titanate-catalyzed preparation of plasticizer esters. The product is treated with aqueous base and then filtered with the aid of a filtration aid, such as bleaching earth, hydrotalcite or magnesium silicate.
WO 97/11048 illustrates the preparation of mixed phthalic esters. The reaction of a phthalic monoester with a polyethylene glycol monomethyl ether is catalyzed with tetraisopropyl titanium. After the reaction has ended, sodium bicarbonate solution is added dropwise. After cooling, 2% water is added, volatile compounds, such as water and solvents, are distilled off under reduced pressure, and the mixture is filtered.
DE 197 21 347 discloses a process for preparing ester plasticizers, in which a mixture of acid or acid anhydride and alcohol is first allowed to react together at from 100 to 160° C. with removal of any water formed, the reaction is conducted to completion with addition of the catalyst and by increasing the temperature up to 250° C., the reaction mixture is reacted with an aqueous alkali metal or alkaline earth metal hydroxide solution, then the excess alcohol is removed, and the remaining raw ester is dried and filtered. The alkaline treatment should appropriately immediately follow the esterification step without preceding cooling of the reaction mixture.